This invention relates to a process for reforming hydrocarbons, and relates particularly to a process for combining high-pressure and low-pressure reforming of naphthas, which eliminates use of a hydrogen recycle compressor in the low-pressure reforming operation.
Reforming is well known as a method for upgrading petroleum gasoline fractions. The art has suggested splitting a relatively wide-boiling-range petroleum fraction into a lighter cut and a heavier cut and separately reforming the two cuts using optimum operating conditions to process each one. Such a process is described, for example, in U.S. Pat. No. 2,767,124, which suggests fractionating a full-boiling-range (100.degree.-430.degree. F) fraction into a lighter (100.degree.-230.degree. F) fraction and a heavier (230.degree.-430.degree. F) fraction, reforming the heavier fraction at a pressure of 5-50 atmospheres and reforming the lighter fraction at a pressure of 1-4 atmospheres.
It is generally necessary to mix hydrogen with a hydrocarbon feed before charging the feed to a reforming reactor. The presence of hydrogen in the reforming reactor at a relatively high hydrogen-hydrocarbon mol ratio has been found to help prevent fouling of the reforming catalyst. Since hydrogen is produced in the reforming reactor in a conventional reforming process, hydrogen in the feed to the reactor is provided by separating the reactor effluent into a liquid product and a hydrogen-containing gas and recycling hydrogen recovered from the reactor effluent. Recycle of hydrogen has required the use of a recycle gas compressor, attendant capital expense and costly operating and utilities expenses being necessitated by use of the compressor. The process of the present invention is directed, in part, to eliminating the necessity for such a recycle gas compressor in a combination reforming process.
The art has suggested elimination of a recycle gas compressor from a reforming process by reforming a single hydrocarbon feedstock in two serial stages, with hydrogen for the second stage being supplied from hydrogen produced in the first stage. An operating scheme of this type is shown in U.S. Pat. No. 2,765,264, which describes a process for reforming a gasoline fraction by: (1) an "aromatization" reaction step, with hydrogen produced being separated from the aromatized hydrocarbon, after which the hydrocarbon is split into several portions; and (2) a "reforming" step in which hydrogen from the previous aromatization step supplies the hydrogen requirement for reforming one of the portions of the aromatized stock. Such a two-stage reforming operation has not found general acceptance in commercial reforming. Thus, compression and recycle of hydrogen remains a serious economic drawback in many reforming operations.
It is known to reform a relatively narrow boiling-range naphtha to provide reformate with high concentrations of benzene and toluene. These aromatics are then separated from the reformate for petrochemical use. It is desirable to reform such so-called benzene-toluene feeds separately from higher boiling hydrocarbons to allow efficient separation of the desired aromatics from the reformate in the absence of unwanted, higher boiling aromatics. Separate reforming systems have thus been used to reform the benzene-toluene feed and the higher boiling hydrocarbons. This has necessitated the use of two complete reforming systems having separate hydrogen recycle arrangements including separate recycle compressors.
Recent catalyst developments in the reforming art have allowed reforming to be undertaken at relatively low hydrogen pressures, e.g., 100-200 psig, with reasonable reforming catalyst stability. Some reforming units can operate at relatively low hydrogen pressures by continuous regeneration of the catalyst, even though it becomes rapidly fouled at a low hydrogen pressure. Likewise, swing reactor process schemes have been used to regenerate reforming catalysts at short intervals when low-pressure reforming processes have been undertaken.